Solstice Stuff
Dec. 20th, 2012 05:42 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
bubbleblowerSome holiday-season poetry:
http://www.well.com/~bubbles/Poetry/BirthdayOfTheLight.txt
http://www.well.com/~bubbles/Poetry/ChristmasCat.txt
See the Sun getting close to the moment of Solstice:
http://www.plergb.com/Analemma/Analemma.shtml
Notice the item labeled "Sun's longitude" in the bunch of numbers and such just above the image. This starts at zero at the March Equinox and increases until it gets to 360 degrees at the next March Equinox, whereupon it starts over at zero.
At the December Solstice the Sun's longitude will pass 270 degrees as it crosses into the sign of Capricorn (Tropical system). (The page is auto-updated approximately hourly, so you may not see anything happen at the actual moment of alignment.)
Northern Hemisphere winter arrives tomorrow (Friday, Dec/21/2012) in the predawn hours (3:12 am California time), the earliest it's been in a long time. This has to do with the Gregorian calendar, with its 400-year cycle.
The actual year is roughly 365 97/400 days long, or a little under 365 1/4. So with every 365-day year the Solstices and Equinoxes are about six hours later than the year before. Since that accumulates to about a day in four years, we put in leap years. A leap year causes the Solstices and Equinoxes to appear a day earlier than they otherwise would, when measured by the calendar.
This is because sticking in February 29 causes March to start a day later than it otherwise would have, so that an instant of time (such as an astronomical event) that would have been on the 21st will instead be on the 20th, as measured by the calendar.
This is all well and good for a few years, but since the correction assumes a year of 365 1/4 days and the actual year is closer to 365 97/400, over time the Solstices and Equinoxes start coming too early. This was the problem with the Julian calendar, and why they had to drop a string of days out when converting to Gregorian.
So the current system drops three leap years every 400 years, for an average year length of 365 97/400 days. Things will swing back and forth a bit, but over time they'll be pretty close. It will be thousands of years before another correction will be needed.
So for now, as a century progresses the Solstices and Equinoxes will swing back and forth on a four-year cycle. After a February 29 they'll be rather early, then they'll be about six hours later each following year until the next February 29.
But each four-year cycle puts them a little earlier than the last, so along about 2096 they'll be really really early. So they make 2100 a regular year (no February 29) to kick then back a day later (as measured by the calendar). Thus by 2103 they'll be extra late, and then when leap years resume with 2104 they'll start ratcheting earlier and earlier again.
They'll be on the early side again in 2196, so again there'll be no February 29 in 2200. Likewise for 2300.
But leaving out one leap year every century is a bit of an over-correction, so they won't do it in 2400. That will be a leap year.
That gives us a 400-year cycle.
In 1903 the Solstices and Equinoxes were about as late as they ever get. Then with every leap year since 1904 they've been getting earlier and earlier. In 2096 they'll be about as early as they ever get. And so on every 400 years, assuming the calendar doesn't get changed between now and then.
Feel free to share all this with your friends.
http://www.well.com/~bubbles/Poetry/BirthdayOfTheLight.txt
http://www.well.com/~bubbles/Poetry/ChristmasCat.txt
See the Sun getting close to the moment of Solstice:
http://www.plergb.com/Analemma/Analemma.shtml
Notice the item labeled "Sun's longitude" in the bunch of numbers and such just above the image. This starts at zero at the March Equinox and increases until it gets to 360 degrees at the next March Equinox, whereupon it starts over at zero.
At the December Solstice the Sun's longitude will pass 270 degrees as it crosses into the sign of Capricorn (Tropical system). (The page is auto-updated approximately hourly, so you may not see anything happen at the actual moment of alignment.)
Northern Hemisphere winter arrives tomorrow (Friday, Dec/21/2012) in the predawn hours (3:12 am California time), the earliest it's been in a long time. This has to do with the Gregorian calendar, with its 400-year cycle.
The actual year is roughly 365 97/400 days long, or a little under 365 1/4. So with every 365-day year the Solstices and Equinoxes are about six hours later than the year before. Since that accumulates to about a day in four years, we put in leap years. A leap year causes the Solstices and Equinoxes to appear a day earlier than they otherwise would, when measured by the calendar.
This is because sticking in February 29 causes March to start a day later than it otherwise would have, so that an instant of time (such as an astronomical event) that would have been on the 21st will instead be on the 20th, as measured by the calendar.
This is all well and good for a few years, but since the correction assumes a year of 365 1/4 days and the actual year is closer to 365 97/400, over time the Solstices and Equinoxes start coming too early. This was the problem with the Julian calendar, and why they had to drop a string of days out when converting to Gregorian.
So the current system drops three leap years every 400 years, for an average year length of 365 97/400 days. Things will swing back and forth a bit, but over time they'll be pretty close. It will be thousands of years before another correction will be needed.
So for now, as a century progresses the Solstices and Equinoxes will swing back and forth on a four-year cycle. After a February 29 they'll be rather early, then they'll be about six hours later each following year until the next February 29.
But each four-year cycle puts them a little earlier than the last, so along about 2096 they'll be really really early. So they make 2100 a regular year (no February 29) to kick then back a day later (as measured by the calendar). Thus by 2103 they'll be extra late, and then when leap years resume with 2104 they'll start ratcheting earlier and earlier again.
They'll be on the early side again in 2196, so again there'll be no February 29 in 2200. Likewise for 2300.
But leaving out one leap year every century is a bit of an over-correction, so they won't do it in 2400. That will be a leap year.
That gives us a 400-year cycle.
In 1903 the Solstices and Equinoxes were about as late as they ever get. Then with every leap year since 1904 they've been getting earlier and earlier. In 2096 they'll be about as early as they ever get. And so on every 400 years, assuming the calendar doesn't get changed between now and then.
Feel free to share all this with your friends.
